Effects of sic buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates

ZnO films have been grown by a sol-gel process on Si (1 1 1) substrates with and without SiC buffer layers. The influence of SiC buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates was investigated. The intensity of the E₂ (high) phonon peak in the micro-Raman spectrum of ZnO film with the SiC buffer layer is stronger than that of the sample without the SiC buffer layer, and the breadth of E₂ (high) phonon peak of ZnO film with the SiC buffer layer is narrower than that of the sample without the SiC buffer layer. These results indicated that the crystalline quality of the sample with the SiC buffer layer is better than that of the sample without the SiC buffer layer. In photoluminescence spectra, the intensity of free exciton emission from ZnO films with the SiC buffer was much stronger than that from ZnO film without the SiC buffer layer, while the intensity of deep level emission from sample with the SiC buffer layer was about half of that of sample without the SiC buffer layer. The results indicate the SiC buffer layer improves optical qualities of ZnO films on Si (1 1 1) substrates.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

Fabrication and characterization of Mg-doped pencil-shaped ZnO microprisms

Two types of novel Mg-doped pencil-shaped ZnO microprisms had been successfully synthesized on Mg(NO₃)₂-coated Si (1 1 1) substrates by thermal chemical vapor deposition method. The as-prepared ZnO prisms were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectroscopy. The straight microprisms are made up of hexagonal pyramids tips and hexagonal prisms bodies. Both of the structures are perfect single crystal and have grown along the [0 0 0 1] direction preferentially. Photoluminescence reveals a red-shift at around 387 nm which is induced by Mg doping and a green light emission peak at around 511 nm. The pencil-shaped ZnO microstructure can provide an improvement in novel ultraviolet light-emitting devices. In addition, the growth mechanism of the special ZnO microprisms is discussed briefly.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Enhanced field emission from ZnO nanopencils by using pyramidal Si(1 0 0) substrates

Zinc oxide nanopencil arrays were synthesized on pyramidal Si(1 0 0) substrates via a simple thermal evaporation method. Their field emission properties have been investigated: the turn-on electric field (at the current density of 10 μA/cm²) was about 3.8 V/μm, and the threshold electric field (at the current density of 1 mA/cm²) was 5.8 V/μm. Compared with similar structures grown on flat Si substrates, which were made as references, the pyramidal Si-based ZnO nanopencil arrays appeared to be superior in field emission performance, thus the importance of the non-flat substrates has been accentuated. The pyramidal Si substrates could not only suppress the field screening effect but also improve the field enhancement effect during the field emission process. These findings indicated that using non-flat substrates is an efficient strategy to improve the field emission properties.     

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

ZnO and Al-doped ZnO(ZAO) thin films have been prepared on glass substrates by direct current (dc) magnetron sputtering from 99.99% pure Zn metallic and ZnO:3 wt%Al₂O₃ ceramic targets, the effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. It shows that the surface morphologies of ZAO films exhibit difference from that of ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (0 0 2). The optical transmittance and photoluminescence (PL) spectra of both ZnO and ZAO films are obviously influenced by the substrate temperature. All films exhibit a transmittance higher than 86% in the visible region, while the optical transmittance of ZAO films is slightly smaller than that of ZnO films. More significantly, Al-doping leads to a larger optical band gap (E_g) of the films. It is found from the PL measurement that near-band-edge (NBE) emission and deep-level (DL) emission are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emission of the thin films is depressed. As the substrate temperature increases, the peak of NBE emission has a blueshift to region of higher photon energy, which shows a trend similar to the E_g in optical transmittance measurement.     

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.     

Large-quantity synthesis of ZnO hollow objects by thermal evaporation: Growth mechanism, structural and optical properties

Synthesis of large-quantity uniformly distributed ZnO hollow objects, i.e. cages and spheres have been performed on Si(1 0 0) and steel alloy substrates by the direct heating of metallic zinc powder in the presence of oxygen. Extensive structural observations revealed that the formed products are crystalline ZnO with the wurtzite hexagonal phases. The Raman-active optical phonon E₂ modes, attributed to wurtzite hexagonal phase of ZnO, were observed at 437 cm⁻¹ for the products grown on both the substrates. The room-temperature photoluminescence spectra showed a broad band in the visible region with a suppressed UV emission, indicating the presence of oxygen vacancies and structural defects in the as-grown structures. Additionally, post growth annealing was also carried out to further investigate the photoluminescence properties of the as-grown products. It was observed that the formation of hollow objects consists of several stages which include the formation of Zn clusters, oxidation on the sheath and sublimation/evaporation of the Zn from the interiors, resulted in the formation of hollow objects.     

45 nm CMOS technology with low temperature selective epitaxy of SiGe

This paper describes the advanced embedded silicon germanium (eSiGe) technologies to apply the 45 nm node CMOS fabrication technology. There are three key techniques as follows. The first technique is a low temperature of epitaxial growth at 550 °C to suppress staking faults in eSiGe layer. The second one is a controlling of recess shape for eSiGe. Sigma(Σ)-shaped recess is applied, because the strain force on the channel of MOSFET is increased effectively by narrowing spacing between source and drain. The third one is to apply particular surface cleaning treatment before the epitaxial growth, to get the excellent SiGe crystallinity. We demonstrated the drain current of I_on = 725 μA/μm and I_off = 100 nA/μm for PMOSFET using above these techniques.     

Low temperature formation of multi-layered structures of ferromagnetic silicide Fe3Si and Ge

Low-temperature (<300 °C) molecular beam epitaxy of Fe₃Si/Ge was investigated. By optimizing growth conditions, Fe₃Si layers with a flat interface and good crystallinity were epitaxially grown on Ge(1 1 1) substrates. In addition, double heteroepitaxial growth of Fe₃Si/Ge on high quality Fe₃Si/Ge substrates was investigated. Reflective high-energy electron diffraction measurements suggested Fe₃Si and Ge layers were epitaxially grown on Fe₃Si/Ge substrates. However, transmission electron microscopy measurements indicated stacking faults formed in the intermediate Ge and top Fe₃Si layers. Improved crystallinity of the intermediate Ge layer is essential to realize high quality [Fe₃Si/Ge]₂ multi-layered structures.     

Effect of surface preparation on the morphology of ZnO nanorods

The effects of Si substrate orientation and surface treatment on the morphology and density of Zinc oxide (ZnO) nanorods were investigated. The size and density of ZnO nanorods were influenced by Si substrate orientation and surface preparation. ZnO nanorods synthesized on the ideally H-terminated Si(1 1 1) prepared with an NH₄F solution resulted in the biggest size and the lowest density. It is suggested that the smoother surface of the Si substrate and lattice shape match with a larger atomic distance result in the increase of the ZnO seedlayer's grain size, which in turn enhances the size of ZnO nanorods grown on it. The optical properties of the ZnO nanorods were affected by their size and crystallinity. The smallest ZnO nanorods with a preferential c-axis orientation synthesized on the HF-treated Si(1 1 1) surface showed the highest intensity ratio of UV to visible emission, and the biggest ZnO nanorods synthesized on the N₂-sparged NH₄F-treated Si(1 1 1) surface showed the lowest intensity ratio of UV to visible emission. Therefore, it can be concluded that Si substrate orientation and surface preparation significantly affect the optical properties of ZnO nanorods.     

Epitaxial growth of InN on nearly lattice-matched (Mn,Zn)Fe2O4

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe₂O₄ (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe₂O₄, which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe₂O₄, which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe₂O₄ (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.     

Effects of crystalline quality on the ultraviolet emission and electrical properties of the ZnO films deposited by magnetron sputtering

The ZnO films were deposited on c-plane sapphire, Si (0 0 1) and MgAl₂O₄ (1 1 1) substrates in pure Ar ambient at different substrate temperatures ranging from 400 to 750 °C by radio frequency magnetron sputtering. X-ray diffraction, photoluminescence and Hall measurements were used to evaluate the growth temperature and the substrate effects on the properties of ZnO films. The results show that the crystalline quality of the ZnO films improves with increasing the temperature up to 600 °C, the crystallinity of the films is degraded as the growth temperature increasing further, and the ZnO film with the best crystalline quality is obtained on sapphire at 600 °C. The intensity of the photoluminescence and the electrical properties strongly depend on the crystalline quality of the ZnO films. The ZnO films with the better crystallinity have the stronger ultraviolet emission, the higher mobility and the lower residual carrier concentration. The effects of crystallinity on light emission and electrical properties, and the possible origin of the n-type conductivity of the undoped ZnO films are also discussed.     

Properties of ZnO thin films grown on Si substrates in vacuum and oxygen ambient by pulsed laser deposition

Epitaxial ZnO thin films have been synthesized directly on Si(1 1 1) substrates by pulsed laser deposition (PLD) in vacuum. The reflection high-energy electron diffraction (RHEED) indicates that streaky patterns can be clearly observed from the ZnO epilayers prepared at 600 and 650 °C, revealing a two-dimensional (2D) growth mode. While the ZnO thin film deposited in oxygen ambient shows ring RHEED pattern. There is a compressive in-plane stress existing in the ZnO epitaxial film, but a tensile one in the polycrystalline film. Compared with the ZnO epilayer, the ZnO polycrystalline film shows more intense ultraviolet emission (UVE) with a small full width at half maximum (FWHM) of 89 meV. It is suggested that the atomically flat epilayers may be powerfully used as transitive stratums to grow high-quality ZnO films suitable for the fabrication of optoelectronic devices.     

Epitaxial ZnO films grown on ZnO-buffered c-plane sapphire substrates by hydrothermal method

ZnO films are hydrothermally grown on ZnO-buffered c-plane sapphire substrates at a low temperature of 70 °C. A radio-frequency (RF) reactive magnetron sputtering has been used to grow the ZnO buffer layers. X-ray diffraction, scanning electron microscopy, and room temperature photoluminescence are carried out to characterize the structure, morphology and optical property of the films. It is found that the films are stress-free. The epitaxial relationship between the ZnO film and the c-plane sapphire substrate is found to be ZnO (0 0 0 1)||Al₂O₃ (0 0 0 1) in the surface normal and in plane. Sapphire treatment, as such acid etching, nitridation, and oxidation are found to influence the nucleation of the film growth, and the buffer layers determine the crystalline quality of the ZnO films. The maximum PL quantum efficiency of ZnO films grown with hydrothermal method is found to be about 80% of single-crystal ZnO.     

Growth of size-tunable periodic Ni silicide nanodot arrays on silicon substrates

The fabrications of size-tunable periodic arrays of nickel metal and silicide nanodots on (0 0 1)Si substrates using polystyrene (PS) nanosphere lithography (NSL) and heat treatments have been investigated. The growth of epitaxial NiSi₂ was found to be more favorable for the Ni metal nanodot arrays. The effect becomes more pronounced with a decrease in the size of the Ni nanodots. The sizes of the epitaxial NiSi₂ nanodots were tuned from 38 to 110 nm by varying the diameter of the PS spheres and heat treatment conditions. These epitaxial NiSi₂ nanodots formed on (0 0 1)Si were found to be heavily faceted and the faceted structures were more prone to form at higher temperatures. Based on TEM, HRTEM and SAED analysis, the faceted NiSi₂ nanodots were identified to be inverse pyramids in shape. Compared with the NiSi₂ nanodot arrays formed using single-layer PS sphere masks, the epitaxial NiSi₂ nanodot arrays formed from the double-layer PS sphere templates exhibit larger interparticle spacings and smaller particle sizes. Since the nanoparticle sizes, shapes and interparticle spacings can be adjusted by tuning the diameter of the PS spheres, stacking conditions, and heat treatment conditions, the PS NSL technique promises to be an effective patterning method for growth of other nanostructures.     

Self-limited growth of Si on B atomic-layer formed Ge(1 0 0) by ultraclean low-pressure CVD system

Utilizing BCl₃ reaction on Ge(1 0 0) and subsequent Si epitaxial growth by SiH₄ reaction at 300 °C, B atomic-layer doping in Si/Ge(1 0 0) heterostructure was investigated. Cl atoms on the B atomic-layer formed Ge(1 0 0) scarcely affect upon the SiH₄ reaction. It is also found that Si atom amount deposited by SiH₄ reaction on Ge(1 0 0) is effectively enhanced by the existence of B atomic layer and the deposition rate tends to decrease at around 2-3 atomic layers which is three times larger than that in the case without B. The results of angle-resolved X-ray photoelectron spectroscopy show that most B atoms are incorporated at the heterointerface between the Si and Ge.     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Influence of growth temperature of TiO2 buffer on structure and PL properties of ZnO films

A series of ZnO films with TiO₂ buffer on Si (1 0 0) substrates were prepared by DC reactive sputtering. Growth temperature of TiO₂ buffer changed from 100 °C to 400 °C, and the influence on the crystal structures and optical properties of ZnO films have been investigated. The XRD results show that the ZnO films with TiO₂ buffer have a hexagonal wurtzite structure with random orientation, and with the increase of growth temperature of TiO₂ buffer, the residual stresses were released gradually. Specially, the UV emission enhanced distinctly and FWHMs (full width half maximum) decreased linearly with the increasing TiO₂ growth temperature. The results all come from the improvement of crystal quality of ZnO films.     

Epitaxial growth of Ba8Ga16Ge30 clathrate film on Si substrate by RF helicon magnetron sputtering with evaluation on thermoelectric properties

Epitaxial Ba₈Ga₁₆Ge₃₀ clathrate thin films were successfully grown on Si substrate by using helicon magnetron sputtering. The (1 0 0) lattice of Ba₈Ga₁₆Ge₃₀ was identified grown on four Si(2 0 0) lattices in small mismatch (0.1%). Both the color of samples and XRD results suggest 600 °C is the optimal substrate temperature for the growth of high quality Ba-Ga-Ge clathrate film on Si substrates. High Seebeck coefficients and electrical resistivities for the deposited clathrate thin films in comparison with those of bulk are obtained. The high crystal quality and thermionic effects in heterostructures may contribute to the larger Seebeck coefficients, while the increasing of interface scattering for electrons probably is the reason for large electrical resistivities. Although the thermoelectric (TE) results are not ideal as designed, our results are significant due to the first successful work on epitaxial growth of Ba₈Ga₁₆Ge₃₀ clathrate thin films on Si substrate with large Seebeck coefficient.